Measuring airflow w/handheld anemometer

I'm a building contractor looking for a way to measure the airflow (in CFM) of the vent fans that are typical in homes. The usual suspect is in the range of 80-150 CFM, has a squirrelcage fan installed in a metal housing that's installed in the ceiling, and a 3" or 4" round outlet that ducted to the exterior (hopefully) with either rigid metal pipe or some sort of flexible pipe. The tool I have available at the moment is this item: http://www.extech.com/instruments/product.asp?catid=1&prodid=2 [Broken]

My hunch is that the specific spot in which the vane is held will have a big effect on the accuracy of the reading. Some fans (when the ceiling grille is removed) have a nice round inlet, but more often than not it's a more complex shape. In addition, it's often not possible to hold the vane directly within the shape, but rather right in front of it.

While googling around I've noticed a few manufacturers of flow hoods, but these seem to be both uncommon for HVAC diagnosis, and very expensive.

I'd be grateful for any comments or advice from those with experience and understanding of this type of issue. My goal is to be able to get a reasonably accurate reading (+/- 10%) of actual airflow. The reason I don't just read the equipment nameplate is that the quality of the ductwork install has a major impact on actual flow.

Staff: Mentor

Welcome to PF.

A balancing contractor will certainly have a flow hood - it is their most important tool.

Measuring airflow at the inlet will be problematic because the air isn't going to be pulled into the fan in a coherent column and will have a tendancy to swirl as it enters. Plus, removing the grille covering it will have an effect on the flow. You're better off measuring at the end of a long, straight section of duct or just at the outlet on the roof.

It is very common to see residential ductwork undersized. The fact that the outlet of the fan is 3" does not imply that the duct should be 3". Fans need a high outlet velocity to properly generate pressure, but once out in the duct, the velocity needs to be lower due to frictional losses....plus, the flex-duct typically used is vastly rougher than hard duct. A 3" flex-duct will only pass 50 CFM (minimum code required toilet exhaust) if it isn't too long and there is a little pressure behind it.